The present invention relates to an automatic support pillar transfer mechanism that arranges pressure-resistant support pillars within an envelope used for a display device which contains field emission cathodes acting as electron sources and to a support pillar transferring method.
In flat display devices such as field emission displays (FEDs) where field emission cathodes are used as electron sources, an envelope formed of thin glass plates is maintained in a high-evacuated state. In order to sufficiently withstand the external pressure, supports called as support pillars are needed between an anode substrate on which display sections are formed with fine pitches and a cathode substrate on which cathode electrodes are formed. In order to support the substrates without disturbing the display, glass fibers, for example, short fibers of an outer diameter of several tens .mu.m are used as the support pillar.
Glass fibers with a smooth cut cross section and a uniform length must be used as support pillars to be erected within an envelope for the field emission display. A conventional process of fabricating support pillars satisfying the above-mentioned requirements will be explained below by referring to FIGS. 3(a) to 3(g).
Plural glass fibers 51 are arranged on the flat glass substrate 52. In concrete, a continuous glass fiber is cut into glass fiber pieces with a predetermined length. The plural cut glass fibers 51 are closely arranged in parallel on the glass substrate 52 (refer to FIG. 3(a)). In this case, the glass fibers 51 are damped with an organic solvent to prevent static electricity.
A bonding agent is applied on the glass fibers 51. Thus, the glass fibers 51 are temporarily fixed on the glass substrate 52. With the bonding agent solidified on the glass fibers 51, the glass fibers 51 are cut at constant intervals using, for example, a cutter with a diamond blade (FIG. 3(b)). Thereafter, the bonding agent coated on the glass fibers 51 is dissolved to remove the cut glass fibers 51 from the glass substrate 52. Thus, a plurality of support pillars 15 of a predetermined length are fabricated (FIG. 3(c)).
The support pillars 15 thus fabricated are arranged using the special jig 53 shown in FIG. 3(d). The jig 53 consists of a positioning plate 54, a cushioning material 55, a porous material 56 and a suction apparatus 57. The positioning plate 54 has through holes 54a corresponding to the arranging positions of the support pillars 15 to be inserted into the substrate. The cushioning material 55 is closely placed on the lower portion of the positioning plate 54 and is formed of a resilient Teflon sheet. The porous material 56 is closely placed on the lower surface of the cushioning material 55 and is formed of a metal such as a stainless steel plate with enormous porosities (e.g. of a diameter of 100 to 300 .mu.m). The suction apparatus 57 sucks and holds the support pillars 15 inserted into the through holes 54a of the positioning plate 54 via both the cushioning material 55 and the porous material 56.
In order to arrange the support pillars within an envelope, one end surfaces of the support pillars 15 are sucked using the suction apparatus 57 to insert them into the through holes 54a. Thus, the one end surfaces of the support pillars 15 are closely held on the surface of the cushioning material 55 by suction while plural support pillars 15 are being inserted into the through holes 54a in an arranged state (FIG. 3(d)). Next, the bonding agent coated substrate 59, over which a bonding agent is coated, is forcibly placed on the one end surfaces of the support pillars 15 in parallel to the positioning plate 54, so that the bonding agent is coated on the one end surfaces of the support pillars 15 (FIG. 3(e)).
The anode substrate 60 used for constructing an envelope is placed under pressure and securely fixed on the ends of the support pillars 15 thus arranged on which the bonding agent 58 are coated (FIGS. 3(f) and (g)). Thereafter, the anode substrate 60, on which the support pillars 15 are securely fixed, is removed from the jig 53. Then a bonding agent is applied to the other end surfaces of the support pillars 15 and the outer fringe portion of the anode substrate 60. An envelope is formed by securely fixing the other end surfaces of the support pillars 15 to the anode substrate and by sealing the spaces between the outer fringe of the cathode substrate and the outer fringe of the anode substrate.
Since the support pillars 15 are evenly transferred onto the entire surface of the substrate forming an envelope for a field emission display (the anode substrate 60 in FIG. 3), it is needed to control the contact height (transfer height) between the ends of the support pillars and the substrate to a suitable value of .+-.10 .mu.m.
Conventionally, as shown in FIG. 3, the support pillars 15 are transferred to the substrate while the condition where the substrate makes contact with the support pillar 15 at the nearest position from the side surface of the substrate is being observed under the microscope 61. At this time, the allowable range of heights of the support pillars 15 to be transferred depends on the cushioning material 55 inserted between the positioning plate 54 and the porous material 56. In concrete, a porous Teflon sheet of an average opening diameter of 3 .mu.m is used to support the support pillars of a diameter of 50 .mu.m. The resilient deformation range of 20 .mu.m of the Teflon sheet provides a transfer allowable range.
However, the above-mentioned art has the following disadvantages.
Since the transfer height of the support pillar 15 is visually controlled under the microscope 61, it is difficult to realize an automatic support transfer operation. In some cases, some support pillars 15 may be transferred to the substrate so that a production failure may occur. It is difficult that the image recognition management using a CCD (Charge Coupled Device) or a computer, evolved from the above-mentioned method, provides a sufficient accuracy and leads to reduced production costs.
Moreover, plural jigs 53 are required for mass-production of display panels. However, because it is difficult to uniformly orient the respective upper surfaces of jigs 53, a variation of about 40 .mu.m is assumed in consideration of the machining accuracy. The gradient of the plane orientation of the substrate is within a tolerance of about 20 .mu.m. The transfer height is out of the allowable range by considering other mechanical accuracy so that a transfer failure may occur. Even when a single jig is used, the above-mentioned error factors limit the transfer range of the support pillar 15 to at most about 12 cm in diagonal line of a screen.